Answer this question truthfully
### Question: Why is the cure rate so varied?
### Response: These are topics for discussion, but errors introduced in the technique can introduce false negative errors. There are numerous reasons why the cure rate is not 100%. Some of Dr. Mohs data revealed a cure rate as low as 96%, but these were often very large tumor, and previously been treated by other modalities. Some authors claim that their 5 year cure rate for primary basal cell cancer exceeded 99% while other noted more conservative cure rate of 97%. The quoted cure rate for Mohs surgery on previously treated basal cell cancer is about 94%. 
1. Modern frozen section method. Frozen section histology does not give the added margin of safety by the cytotoxic Mohs paste,  originally used by Dr. Mohs. This paste might have destroyed any residual cancer cells not detected by the pathologist.
2. Missing epidermal margins. Ideally, the Mohs section should include 100% of the epidermal margin, but greater than 95% is often accepted.  Unfortunately, vigorous scrubbing, poorly controlled initial curettage, poor tissue health, technician's error, and surgeon's error can introduce areas missing epithelial margin. Some surgeon consider 70% epithelial margin acceptable, while other suggests 100% margin. In the ideal situation, 100% of the epithelial margin should be available to be reviewed on serial sectioning of the Mohs specimen.
3. Misreading of the pathology slide. It is difficult to differentiate between a small island of basal cell carcinoma and a hair follicle structure. Many Mohs surgeon limits their tissue processing to include only 2 sections of tissue.  This severely hampers their ability to determine if a structure is a hair follicle or a carcinoma. Two histologic sections can not fully distinguish these two nearly identical structures,  and can lead to either "false negative" or "false positive" errors by either calling a section clear of tumor, or calling a section positive for tumor, respectively. Serial sectioning of the tumor is preferred by other surgeons.  Surgeons who performs serial sectioning through the block of tissue (usually 100 micrometres apart) are assured the contiguous nature of his tumor, the distance of the tumor from the surgical margin, and is familiarized with the nature of the tumor. Serial sectioning also makes it easier to work with three dimensional tumor with difficult to compress margins.
4. Compression artifact, freezing artifact, cautery artifact, tissue folds, crush artifact from forceps, relaxing incision artifact, cartilage dropping out, fat compression, poor staining, dropping of tumor, etc.  These can be introduced as the tumor is "flattened". Stain can run from the surgical edge, and stain the surgical margin - giving a false impression that the entire surgical margin is clear, when it is not. While some surgeon unfamiliar with the "whole piece" or "PacMan"  methods of processing might suggest that multiple piece sectioning is better than one. The more tissue sections are cut, the more artifacts in staining and tissue malformation will be introduced. It is imperative for the surgeon to be fully familiar with tissue handling and processing; and not simply relying on a trained technologist to perform his sectioning.
5. Hard to see tumor in heavy inflammatory infiltrate.  This can occur with squamous cell carcinoma, especially when complicated with local infection, or intrinsic lymphoproliferative disorders (chronic lymphocytic leukemia). Because of abnormal peripheral blood profile, response to inflammatory skin conditions with patients with myelomonocytic leukemia can have appearance of atypical cells at sites of inflammation, confusing the Mohs surgeon. 
6. Perineural spread, and benign changes simulating perineural spread. Tumor spreading along a nerve can be difficult to visualize, and sometime benign plasma cells can surround the nerve, simulating cancer. 
7. Difficult to cut and process anatomical area.  Examples would be the ear, and other three dimensional structures like eyelids. The ability to make a scallop shaped incision is increasingly difficult when the surgical surface is no longer a flat plane, but is a three dimensional rigid structure.
8. Recurrent skin cancer with multiple islands of recurrence. This can occur with either previous excision, or after electrodessication and curettage. As these residual skin cancer are often bound in scar tissue, and present in multiple location in the scar of the previous surgical defect - they are no longer contiguous in nature. Some surgeons advocate the removal of the complete scar in the treatment of "recurrent" skin cancers. Others advocate removing only the island of local recurrence, and leaving the previous surgical scar behind. The decision is often made depending on the location of the tumor, and the goal of the patient and physician.
9. Unreported or underreported recurrence. Many patients simply will not return to the original surgeon to report a recurrence. Consulting surgeon on the repeat surgery will often not inform the first surgeon of the recurrence. The time it takes for a recurrent tumor to be visible to the patient might be 5 or more years. Quoted "cure" rates must be looked upon with the understanding that a 5 year cure rate might not necessary be correct. As basal cell carcinoma is a very slowly progressing tumor, a 5 year no recurrence rate might not be adequate. Longer follow up might be needed to detect a slow growing tumor left in the surgical scar.
10. Poor training of the surgeon/pathologist/histotechnologist. While Mohs surgery is essentially a technical method of tissue handling and processing, the skill and training of the surgeon can greatly affect the outcome. The house of card is built with a foundation of good tissue handling, good surgical skill and hemostasis, and the bricks are the tissue processing and staining technique. A surgeon without a good histotechnologist is a house built with straw, and a histotechnologist without a good surgeon can not produce quality slides. While originally, surgeons learned the procedure by spending a few hours to several months with Dr. Mohs;  today, surgeons complete residency and fellowship spending hundreds of hours observing and doing Mohs surgery. It is highly encouraged that a physician interested in learning Mohs surgery should spend extended time observing, cutting, processing, and staining Mohs specimens. The histology block should be correctly mounted and cut the first time, as there is no second chances in Mohs histology. It is not a procedure that can be taught or learn in one weekend. Many residency and Mohs fellowship continue to teach the processing of only 2 Mohs sections per tumor. 
Irrespective of the problems associated with Mohs surgery, a true cure rate approaching 100% can occur with primary basal cell carcinoma (previously untreated) if proper respect of the physician's limitation, the procedure's limitation, and his laboratory staff's limitation. Conservative approach such as serial sectioning, good staining technique, and conservative Mohs margin (example: tumor at least 200 micrometre from the surgical margin) can assure the lowest recurrence rate.